feat: 닉네임 중복 확인 메서드 추가

Author: MEOHIN

src/main/java/com/back/domain/user/repository/UserRepository.java

@@ -6,5 +6,6 @@
 
 @Repository
 public interface UserRepository extends JpaRepository<User, Long> {
+    boolean existsByNicknameAndIdNot(String nickname, Long id);
 }